CHEM 128B

We know the overwhelming feeling CHEM128B is at this point of the semester. Too many reagents and reactions to remember... and they seem endless. But, these two reaction sheets will help you by having the necessary info in one place. Now, for your to store this information in your brain, try making flash cards. Perhaps, write the reactants on one side, and the ending result on the other....

However, if you want to fully know what happens, because in your test you will have to draw mechanisms.... this is where asking a tutor for help comes into place. Try to understand the mechanism behind each reaction, why is that the ending result? Why do we start with a carboxylic acid or acid chloride? LET'S PRACTICE TOGETHER, and master these reactions together! :)

Remember, that practice makes perfect!

Lizbeth Monthly Project #3 - 11/29/2017

Lizbeth Monthly Project #2 --10/18/17

Epoxide Reaction Stereochemistry

Added by Magdalena Hoskins, Fall 2024

Utilize this summary of the reactions of alcohols when doing your homework or studying for an exam.

To help your brain retrieve this reactions, it will be helpful if you rewrite them.

Once you feel confident with the topics, ask a tutor for practice questions, so you can test your knowledge and allow us to help you fill in those gaps of knowledge you might have.

Lizbeth Monthly Project #1 9/29/17

Aromatic Compounds and Reactions

Added by Hend Ali, Spring 2025

Structure and stability of Benzene

Benzene is a six-carbon ring that appears to have alternating double bonds, but is instead best represented by a resonance hybrid. The delocalization of π-electrons across the ring make benzene very stable, which is known as aromaticity.

Key Takeaway: Benzene does not behave like a typical alkene in a reaction as it resists addition and prefers substitution, to perserve its aromatic nature.

How a benzene ring can be depicted
1) Lewis Structure

2) Hexagon with 3 π bonds

3) Hexagon with π electron delocalization

Hückel's rule for Aromaticity

To determine whether a compound is aromatic, it must meet Hückel's rule:

It must be cyclic
It must be planar (all atoms in the ring must be sp^2 hybridized or conjugated)
It must have a continuous ring of p orbitals
It must contain (4n+2) π electrons, where n is an integer (0,1,2,..)

Electrophilic Aromatic Substitution (EAS)

Aromatic rings undergo substitution reactions instead of addition to preserve aromaticity. In EAS, an electrophile replaces a hydrogen atom on the aromatic ring.

General Mechanism:

Formation of the electrophile: often requires a Lewis Acid catalyst like AlCl₃ or H₂SO₄
Attack of the aromatic ring on the electrophile creates the formation of a carbocation intermediate (called the sigma complex)
Deprotonation to restore aromaticity

Examples of EAS Reactions

Reaction Electrophile Lewis Acid

Nitration NO₂ H₂SO₄

Halogenation

Bromination Br AlBr₃ or FeBr₃
Chlorination Cl AlCl₃ or FeCl₃

Sulfonation SO₃H H₂SO₄

Friedel-Crafts Alkylation R AlCl₃ or FeCl₃

Friedel-Crafts Acylation RCO AlCl₃ or FeCl₃

Activating vs. Deactivating

Substituents on the benzene ring affect both the rate and position of substitution.

Activating Groups make the ring more reactive:
- Electron-donating groups
- Examples: –OH, –OCH₃, –NH₂, –CH₃
- Direct to ortho/para positions
Deactivating Groups (mkae ring less reactive)
- Electron-withdrawing groups
- Examples: –NO₂, –CF₃, –SO₃H, –COOH
- Direct to meta position

Goals this month with the Chemistry tutors:

Translate IR, NMR, and Mass spectra to compounds and vice-versa
Draw a compound given its molecular formula and IR, NMR, and/or Mass spectra
Differentiate between functional groups on an IR, NMR, or Mass spectra
Predict the peak range of a given functional group
Discuss how to study for this topic:
- - How to make flash cards for the IR, NMR, and Mass spectra peaks
  - ask the tutor for tips for when working on IR, NMR, and Mass Spectra
  - Ask about the best approach to take and study from practice tests

Activities to do at home:

Create flash cards for the IR values and Chemical shifts you must remember
Write a list of the values (IR & Chemical shifts) for retrieval when doing homework.
Do practice problems or homework, first without looking at your list. ONLY use your list if you need to correct your answer.
summarize the lecture into 5 sentences
use the provided links

Practice With Identifying the Compound that Corresponds to an IR Spectrum

Shared by manharsh & Viktor 08/16/2017

Challenge question: What is the difference between the fingerprint and diagnostic regions? Which one is more useful?

IR spectroscopy is one of the first things introduced in chem 128B and is an extremely important concept throughout the semester.

After watching this video, try making your own molecules and correspond the correct peaks with them.

https://webspectra.chem.ucla.edu/irtable.html
- you should bookmark this site as you will be referring back to it a lot throughout the semester and in lab
- Memorize the peak ranges and intensity

NMR in 14 Minutes

Shared by Viktor & Manharsh 08/16/2017

Once you're done watching the video, ask a tutor for a practice problem & let's put to practice what you just learned. Let us guide you through the process. :)

Mass spectroscopy is a confusing topic, but this video will make it a lot more simple.

Taking notes while watching the video will help you to recall more information, once it's over. Try to answer the following questions while watching the video.

What's the chemical shift?
What's the difference between up-field & down-field?
What does integration tell you?
What's the "n+1" rule?

Lizbeth 8/22/17

NMR Peak Table

Shared by Parveen Kaur (8/17/2017)

Shared by Viktor & Manharsh 08/16/2017

A good way to fully grasp the information given in the video, try to work out the same problem he's explaining in the video.

Spectrometry tends to be a confusing topic fr many students. This video is meant for you to better understand this topic.

Remember to take notes while watching this video. Have questions in mind that you would like the video to answer, if you can't fully understand it, Let a tutor know and we will help you out.

Lizbeth 8/22/17

Chemical Spectra

Note: The order presented may not be the same for all professors

Use the diagram below to help you solve NMR Spectroscopy questions.

Shared by Rojina N. 1/26/20

If you are confused about NMR peaks and splitting patterns, watch the following video to help you answer relevant questions.

Taking notes while watching the video will help you to recall more information, once it's over.

Shared by Rojina N. 1/26/20

These videos include walkthroughs of how to predict the structure of a molecule from its H-NMR spectrum. First, try to determine the structure on your own, then watch them solve the problem and check your work!

Magdalena Hoskins, Fall 2024

NMR Splitting Patterns

Shared by Rojina N. 1/26/20